Android: Sometimes a carrier will blacklist a phone's ESN (electronic serial number) because it was stolen or if the phone was disconnected due to nonpayment. In these cases trying to activate the phone with the same or a different carrier will be unsuccessful and can bring police involvement if the phone was stolen. Save yourself from this worry by using Android app PocketESN.

There are free and paid versions of the app. With the free version you can only check the ESN of the phone on which the app is installed, and you'll need an internet connection for this (Wi-Fi is fine). The paid version ($5.99) allows you check any phone's ESN.

The app itself is pretty straightforward; input the ESN (or just tap the My ESN button if you're checking the phone you're using) and select a carrier. You'll know in a few seconds if the ESN is clean or not.

If you do a lot of phone sales and upgrades, six bucks seems like a small price to pay if you're worried about buying stolen merchandise. Think of it as CarFax for your phone.

PocketESN is available in free or pro versions from the Google Play Market (formerly Android Market)